modulation shapes

[WeAreAs1 at aol.com]

In a message dated 10/24/98 9:00:30 AM, jhaible at metronet.de wrote:

<<That's something I have not yet tried, but it sounds reasonable.

I guess at the low turning point a pitch change effect dominates, and a

sharp corner in the mod signal would cause an unpleasantly

strong pitch change, while on the high end the filter effect is dominating,

and a round corner would just make the filter stay too long in a

rather uninteresing position of the comb filter.

I would not have found that myself, but once I've read about such a 

modulation shape, it's quite reasonable. Who did invent it, btw. ?>>

Craig Anderton used a similar LFO waveform in the flanger kit that he designed
for PAIA back in the mid-1980's.  His LFO was made with a CEM 3340 VCO.  He
referred to the waveform as a "Hypertriangle".  I don't think it had the
"round bottom/sharp top" shape of Juergen's experimental LFO, but it was
designed so that its would frequency would speed up at its waveform peak and
slow down at its valley.  According to Craig, this would result in a smoother,
more natural sounding flanger modulation.  I don't know - I never got a chance
to hear one in action, nor have I ever seen the schematic.

I imagine that he was acheiving this waveform by simply feeding back a small
part of the VCO's triangle wave into its CV input.  As the wave increased in
DC amplitude, this feedback would cause the VCO to speed up, and then slow
down as the wave's amplitude decreased.  The resulting waveshape was probably
like a triangle with a semi-exponentially rising and falling shape.

I guess you could get such an LFO waveform to also have the "rounded bottom"
by putting just one diode to ground across its final output, as seen in some
typical diode-based triangle-to-sine converters and diode-based fuzztones
(these circuits always have two diodes, oppositely polarized).  You'd just
have to get the waveform's amplitude and DC offset just right to get the
lowest point of the wave in the proper range for the diode to to work.

Of course, I doubt that very many of us would be willing to sacrifice a
precious 3340 to such proletariat use!  Does anyone on the list have one of
those old PAIA flangers?  I'd like to get a copy of the schematic someday. 

Michael Bacich